90° tight 3 dB couplers are key components in monolithic microwave integrated circuit (MMIC) technology. They are commonly used in the design of frequency converters, balanced amplifiers and modulators. For planar microstrip MMICs, there are few alternatives and 90° couplers are generally implemented through the use of Lange couplers or Branch-line couplers. Because of their large size such couplers are responsible of an important and irreducible part of the MMIC cost.
Recently, the introduction of the multi-layer or three-dimensional MMIC technology enabled the development of broadside couplers fabricated using transmission lines on top of each other and separated by a thin dielectric layer. Due to the multi-layer nature of broadside couplers, the thin film transmission lines can be folded in a meandering way to minimize the overall size.
Several configurations of Broadside coupler have been proposed in the literature: In Japanese Patent Document. No. 405037213, I. Toyoda et al. proposed a broadside coupler constructed with two conductor strips on different layers and a ground metal below the conductor strips. Details of this broadside coupler can also be found in the 1992 IEEE publication “Multilayer MMIC branch-line coupler and broad-side coupler” at pages 79–92.
The proposed topology enables tight 3 dB coupling but requires optimization of the polyimide layer thickness between the two conductor strips and to the ground resulting in little design flexibility. The design of such a coupler is usually done using electromagnetic simulation software and typically requires several optimizations to achieve optimum performance. Moreover, the control of the coupling factor is limited and tight 3 dB coupling can only be achieved for a specific value of the ground plane to respective conductor strips ratio of the polyimide layer thickness. Insertion loss is quite high in this case (˜2 dB) due to the presence of the ground plane close to the conductor strips. Also, because of the ground plane being close to the conductor strips, isolation between the output coupled port and the output direct port is only −15 dB.
In 1994, Mernyei et al. demonstrated a new broadside-offset coupler. The device is the combination of the coplanar waveguide (CPW) line formed on a first metal layer and a microstrip (MS) line on a second metal layer above the first metal layer. A 5 μm thick polyimide layer separates the two metal layers.
The coupling in the structure is controlled by the offset spacing of the MS line above the CPW line and ranges between −3 dB and −30 dB. However, because of the CPW nature of the lower line, the characteristic dimensions of the coupler are large and the lines are unlikely to be foldable in a meandering way.
In 1996, M. Engels and R. H. Jansen proposed to realize broadside couplers (so called “quasi-ideal coupler”) using three metal layers as shown in FIG. 1. First conducting strip 120 is formed on first dielectric layer 150. Second conducting strip 110 is formed on second dielectric layer 140. Ground plane 131, 132 with a gap therein is formed between second dielectric layer 150 and substrate 160. In the same manner as the coupler proposed by I. Toyoda et al. the ground plane is placed on top of the substrate so that no backside processing is necessary.
Because of the difference of material surrounding the conductor strips, the analysis of the proposed topology refers to the analysis of asymmetrical coupled lines in inhomogeneous media. The characteristic dimensions (w1, w2, S, Sgnd and h1 all normalized to h2) can be deduced from the resolution of the well-known mode parameters equations in inhomogeneous media.
The ground plane below the two conductor strips is open with a gap Sgnd from the larger conductor strip to provide an additional degree of freedom so that the postulate of the mode parameter relation in inhomogeneous media can be satisfied.
M. Engels and R. H. Jansen didn't demonstrate experimentally the proposed concept. However, the optimization of the mode parameters using a quasi-static analysis ignoring frequency dispersion and loss, and assuming conductor strips of zero thickness, shows that potential ideal performance in term of input/output reflection and isolation can be achieved.
This result is particularly attractive. However, for the design of most MMIC involving a quadrature coupler, phase and amplitude balance prevail over any other characteristic. For example, the degradation of phase and amplitude imbalance has dramatic effect on the LO suppression ratio of a quadrature up-converter.
For the broadside coupler proposed by M. Engels and R. H. Jansen, the ideal quadrature between the coupled and the direct port cannot be achieved and the phase imbalance deviates linearly with the frequency. At millimeter-wave frequencies, the phase imbalance can be significant and strongly limit the use of such a broadside coupler.
M. Engels and R. H. Jansen proposed to compensate the phase dispersion by connecting a transmission line to both ports of one of the conductor strip. However, this contributes to the degradation of the amplitude imbalance and compactness of the coupler.
Therefore, the prior art has limitations that the present invention seeks to overcome.